Conventional methods for checking outgassing of fuel from engine oil of an internal combustion engine include that described in DE 10 2007 046 489 B3, which describes a method of this kind in which operating parameters of the internal combustion engine are detected, with a mass flow of fuel from the crankcase to the intake section being determined as a function of detected operating parameters. The internal combustion engine is controlled or monitored as a function of the mass flow of fuel from the crankcase to the intake section. Primarily, uncombusted fuel can be dissolved in a lubricant of the internal combustion engine immediately after a cold start of the internal combustion engine, said uncombusted fuel then evaporating again as the operating temperature rises. The dissolution of the fuel in the lubricant causes an undesired change in the lubricating properties of the lubricant. The fuel which is dissolved in the lubricant evaporates as the operating temperature rises, and collects primarily in the crankcase in a reciprocating-piston internal combustion engine. In order to prevent uncombusted fuel being emitted to the environment, the crankcase is connected to the intake section by means of a crankcase venting system. A mass flow from the crankcase to the intake section, which mass flow is dependent on the operating state of the internal combustion engine, is established on account of a pressure drop from the crankcase to the intake section. This mass flow comprises exhaust gas and air which are routed from the combustion chamber into the crankcase past the sealing rings of the pistons, and possibly fuel which is evaporated from the lubricant in the crankcase.
The control system of a modern internal combustion engine monitors the ability of the components of said internal combustion engine to function by means of diagnosis of the operating parameters which are available to said control system. Fuel which evaporates from the lubricant and is routed into the intake section via the crankcase venting system enriches the fuel/air mixture in the combustion chamber or chambers of the internal combustion engine. For complete combustion of the fuel and the atmospheric oxygen (λ=1), the control system of the internal combustion engine has to meter less fuel in comparison to the fresh air which is supplied to the internal combustion engine. A deviation of this kind is interpreted by the control system as a defect in the internal combustion engine, for example a fuel supply device, or in a λ sensor. In order to prevent incorrect interpretation, the situation of an excessively low quantity of fuel which is to be metered to the internal combustion engine over a predetermined period of time after a cold start is usually not interpreted as a fault. As a result, diagnosis of a defect in the internal combustion engine is significantly limited. The limitation has particularly serious consequences if the internal combustion engine is always operated only for a short time, for example in city traffic.